In this field, coral has been proposed and has been widely used for ten years or so. This natural material has a mineral exoskeleton of open porosity formed, in the main, of aragonite. Aragonite is an allotropic form of calcium carbonate, which crystallises in an orthorhombic system with lattice parameters a=4.9623 (3).ANG., b=7.968 (1).ANG., c=5.7439 .ANG. for Z=4. Its theoretical density is d=2.93.
Within the state of the technology, it has been proposed to use preparations based on coral as bone reconstitution materials within the field of paradontology (paradontal diseases, bone surgery correcting loss of substance). This material is used in this context in the form of granules of size ranging from 300 to 450 gm for the filling in of paradontal defects and from 600 to 1000 pm for the filling of sockets after dental extractions. Granules are preferred to blocks which lead to the exfoliation of the coral fragment after its implantation, the size of the particles recommended for the filling in of paradontal defects then being from 300 to 500 pm.
French patent application FR 2637502 describes a bone reconstitution material made up of madreporous coral washed of any original organic substances and containing an organic osteogenesis accelerator in the form of a proteinic gel based on Type I collagen.
International patent application WO 94/26322 discloses a porous material made up of a coral skeleton (of the Porites, Acropora, Goniopora, Lobophyllia, Simphyllia or Millipora type) and a growth factor capable of being an osteo-inductive agent.
European patent application EP 395187 describes a bio-material made up of coral coated with a layer of hydroxyapatite.
Experience has shown that coral has a high bio-compatibility that confirms that it is of interest as a material for bone reconstitution.
However, such a material has the major disadvantage of high cost brought about by its processing when it is to be used as an implant material. In practice, coral gathered from nature must be divided up, then washed and incubated for several days in products such as sodium hypochlorite in order to remove all organic substances from it. Such an incubation must be carried out in such a way that the product used is able to penetrate into the entire structure of the porous coral. Blocks of coral treated in this way must then be broken up and sorted into granule size ranges, or machined in order to be made geometrically suitable for the implantation sites and then washed once again.
Furthermore, coral has a porosity that it is not possible to control. In relation to the desired speed of resorption, it is advisable to choose this or that type of coral in relation to its natural porosity.